The present invention relates to the production of alumina and, more particularly, to the production of an alumina of high purity.
Currently, a widely used processes for the production of alumina involves reacting aluminum or an aluminum bearing material with an olefin such as ethylene to produce a trialkyl aluminum. The trialkyl aluminum is distilled to remove impurities, and the purified triethyl aluminum reacted with an alpha olefin such as ethylene to produce what is known as an aluminum alkyl growth product, the chain length of the alkyl groups depending upon the degree of reaction between the olefin and the trialkyl aluminum. Subsequently, the aluminum alkyl growth products are subjected to a two-stage oxidation. In the first stage, the aluminum alkyls are oxidized approximatey 40% to aluminum alkoxides. In the second stage, the oxidation of the aluminum alkyl growth product is carried to completion, the second stage normally being carried out in the presence of a titanium compound such as a titanium halide to suppress side reactions and increase the yield of the aluminum alkoxides. The aluminum alkoxides (oxidized growth product) from the oxidation are then hydrolyzed with water to produce alumina and alcohols. The reaction is normally carried out in a large excess of water so that an aqueous slurry containing roughly 12% by weight alumina is formed. The alcohols, which generally are of a relatively long chain, form a separate phase and are removed by decanting from the bulk phase and then steam stripping the aqueous slurry containing the alumina to remove the residual, dissolved alcohols. The alumina is recovered by drying the slurry in a spray dryer.
In the above-described process process for the production of alumina, there are several stages in the process at which impurities or by-products can be introduced or formed and which end up in the alumina end product. For example, an obvious source of contamination of alumina is the titanium which is added to suppress side reactions in the second stage of the oxidation reaction of the aluminum alkyls growth product. It is common for alumina produced according to the above-described process to have a titanium content (as TiO.sub.2) of from about 1600 to about 1900 ppm. Such high contents of titanium can alter the electrical and optical properties of the alumina making it unsuitable for forming into high technology items which require extremely pure alumina.